An aircraft fuselage is typically constructed as a series of longitudinally spaced circumferential frame members which define the general latitudinal (cross-sectional) fuselage shape, and a series of spaced stringer members running longitudinally with respect to the aircraft fuselage which contribute to the stiffness of the external skin. Together, the frames and stringers constitute a robust internal structure that provide support to the external fuselage skin.
The aircraft fuselage skin is formed of a single piece or separate pieces joined with skin splice straps, covering the various members of the internal structure. The frames and stringers are attached to the skin using different techniques. Usually, the stringers are joined to the skin pieces in an earlier stage of the manufacturing cycle, using rivets or adhesive, to form stiffened skin panels. In a later stage, the stiffened skin panels are spliced and attached to the frames. For ease of production, the frames can be fabricated in segments, in the same quantity as of the stiffened skin panels. The corresponding segments of the frames can be installed onto each stiffened skin panel, forming framed stiffened skin panels. In a next stage, the framed stiffened skin panels can be assembled, and the skins and frames are spliced.
Three different strategies have previously been adopted to integrate the stiffened skin panels and the frames. In one of the strategies, the frame is fastened directly to the stiffened skin panel, using rivets or adhesive. In another strategy, an additional member termed shear tie (or shear clip, or shear cleat) is used, with separate fasteners for the frame and for the stiffened skin panel. An adhesive can be used instead of rivets. The shear ties can be installed to the stiffened skin panel during the same stage wherein the stringers are installed, or in an intermediate stage before the frames are installed. In accordance with a third strategy, the frames are attached to the stiffeners of the stiffened skin panel, without either a direct or indirect attachment to the skin of the stiffened panel.
Whichever the strategy may be adopted, it is important to ensure that the installation of the frames and stringers must be accomplished without structural interference. In this regard, since the stringers are subjected to high axial loads, they therefore must be continuous along the longitudinal extent of the aircraft fuselage. On the other hand, the frames are subjected to high bending loads and therefore must be designed for good depth, to provide a high moment of inertia. Because the frames are designed for deep cross-section, they can accommodate apertures (colloquially terms “mouse holes”) to allow for passage of the stringers therethrough.
The aircraft fuselage can be constructed using materials of two distinct groups, namely metallic materials (most commonly aluminum) and composite materials (most commonly laminates of carbon-fiber reinforced epoxy). In previous aircraft designs, only materials pertaining to the same group (either metallic or composite) have been used to manufacture the entire aircraft fuselage structure including skins, stringers and frames. Several viable design reasons for not constructing an aircraft fuselage from metallic and non-metallic materials include: (a) dissimilar materials are prone to corrosion; (b) the joint of a metallic member to a composite member is more complex to accomplish; (c) high interface loads can be generated by temperature changes, within the operational envelop; (d) the technologies involved in the production of metallic parts are very different than those required to fabricate a composite structure.
From the viewpoint of electrics and electronics engineering, the metallic structures have wide acceptance, because metals are electrically conductive. This characteristic is not found in the composite materials, since they have poor or no electric conductivity. On the other hand, composite materials offer significant advantages in terms of weight and manufacturing cost, especially when the design is tailored to the peculiar ability of the composites to integrate the structural members, drastically reducing the quantity of fasteners. If the aircraft body is constructed in composite materials, special details are required, such as the inclusion of a wire mesh ply extending throughout the outer surface of the body skin so as to protect the structure against the impact of electrical discharges commonly encountered in the atmosphere. Additional wires are necessary to replace the natural electric path, represented by the metallic structure. This electric path is useful to connect the electric power generators and batteries to the electric consumers, including lights, motors, computers and other units.
According to known aircraft manufacturing techniques, a complete aircraft fuselage can be fabricated from non-metallic composite materials, with stringers and skins fabricated from similar laminate materials. Such a technique thereby allows all such components to be co-formed with one another, that is to be processed in a single heating cycle inside an autoclave so as to cure the polymeric resin from which the laminates are made. Before entering the autoclave the skins are laid over the stringers, forming a continuous seamless part. After the cure cycle, in the resulting stiffened skin, the stringers and skins are joined by the resin as if they had been bonded. In such an aircraft fuselage structure, the frames are typically separately fabricated from a composite material and are subsequently joined to the skins with shear ties, also fabricated separately from a composite material. The shear ties are fastened to the stiffened skin using a mechanical system based on pins similar to bolts and locking parts similar to nuts. The same system is used to fasten the frames to shear ties. It remains evident that this design has not taken full advantage of the cost-saving and weight-saving gains of the composite material, since too many fasteners are used and the assembly presents low integration level. Like any other all-composite fuselage structure, this conventional design technique requires the installation of additional metallic details to perform the electrical and electronic functions which involve further weight and cost accretion.
In such a composite aircraft fuselage, the stiffened skin is fabricated by wrapping separate mandrels with a laminated tape. Upon curing of the resin, the resulting cylindrical trunks receive the frames. Subsequently, circumferential straps are employed to splice the trunks with butt-type joints, thereby forming the aircraft body assembly. The butt straps are separately fabricated using the same composite material used for the skins. This system relies upon a set of dimensional constrains indicated in the manufacturing drawings to prevent the misalignment of the trunks, which can ruin the assembly of the fuselage.